Alternate bearing (also called biennial or uneven bearing) is the tendency of a perennial crop plant to produce a heavy crop in one year (on-crop, on-year) followed by a light crop or no crop (off-crop, off-year). During the on-crop year, there is a high yield of small fruit with reduced market value, while during the off-crop year, the fruit size is good but there are too few fruit to provide the grower with a good income after covering production costs. This leads to price instability, erratic annual returns to growers, and possible loss of market share. The lack of crop in the off-crop year also compromises the development or sustainability of value-added products industries.
The alternate bearing phenomenon is widespread, occurring in both deciduous and evergreen trees, as well as in both perennial fruit and nut crops. It may occur over an entire region, in a block of trees, in an individual tree, or even in part of a tree or one branch. This problem is of significant economic consequence in apple, apricot, avocado, citrus, especially mandarin, peach, pecan, pistachio and plum to name a few crops of commercial value. Approximately every other year, grower income is reduced below the orchard's potential by production of an off-crop. Therefore, there is a recurring need to mitigate alternate bearing.
Alternate bearing is initiated by climatic conditions (e.g., freeze damage, lack of chilling, low or high temperatures during flowering or fruit set) that cause poor flowering or excessive flower or fruit drop, resulting in an off-crop. This is followed by an on-crop, typically the next year, depending on how long it takes for the trees to recover. Conversely, climatic conditions that are optimal for flowering and fruit set such that crop thinning fails to take place result in an on-crop that is followed by an off-crop. Once initiated, alternate bearing becomes entrained through the effect of crop load on endogenous tree factors that ultimately impact floral intensity, such that the heavy on-crop reduces return bloom the following spring, whereas the light off-crop results in an intense return bloom the next spring. Because alternate bearing is initiated by climate, the need for a corrective strategy is recurring.
Fruit is the main tree factor causing alternate bearing, and the number of fruit on a tree is the main factor influencing floral intensity of the return bloom. The presence of a large number of fruit (heavy on-crop) reduces the number and length of summer and fall vegetative shoots in a variety of perennial fruit and nut crops, thereby reducing number of shoots and nodes on shoots that can produce inflorescences in the spring (Verreynne, Ph.D. Thesis, University of California Riverside, 2005; and Verreynne and Lovatt, J Amer Soc Hort Sci, 134:299-307, 2009). The presence of a large number of fruit on the tree in spring also inhibits spring bud break, further reducing the number of floral shoots and flowers that develop in spring. In some crops the fruit is not mature and cannot be harvested until after spring bloom, but for others, the presence of the mature crop on the tree in spring unnecessarily increases the degree of alternate bearing (Verreynne, Ph.D. Thesis, University of California, Riverside, 2005; and Verreynne and Lovatt, J Amer Soc Hort Sci, 134:299-307, 2009).
Current strategies to mitigate alternate bearing require reducing production in the on-crop year. This may be accomplished by inhibiting flowering, or removing flowers or young fruit chemically, by pruning or by hand. As such, what is needed in the art are compositions and methods to mitigate alternate bearing that are safe without being labor intensive and that do not reduce yield in the on-crop year.